1. The Field of the Invention
This invention relates to antennas and, more particularly, to novel apparatus and methods for providing a broadband roll-out antenna system which is portable, resistant to destruction, and easily deployed.
2. The Prior Art
The design and manufacture of antennas for transmitting and receiving electromagnetic signals has been the subject of ongoing research for many years. Today, a multitude of different antennas are available for a wide variety of applications. In choosing among these many different antenna designs, a number of different factors must be considered.
It is first necessary to consider whether the antenna will transmit and receive electromagnetic signals which have the appropriate frequency. To a large extent, this will depend upon the specific needs of the antenna user and upon the particular conditions under which the antenna will be operated.
For example, television crews which are filming on location, or military troops may desire to transmit signals to a station which is not in their line of sight but which is less than about 300 miles away. In such cases, it will generally be necessary to transmit and receive signals having a frequency within the range of from approximately 2 to approximately 16 megahertz (MHz). The electromagnetic signals must be launched at "near vertical incidence" (that is, nearly vertically upward), in order to be reflected off the ionosphere back to the receiving station at this short distance.
When transmitting and receiving electromagnetic signals over a distance greater than approximately 300 miles, signals of higher frequency will often be required. In addition, the antenna employed must be capable of launching the electromagnetic signals at an angle which is near the horizon.
It may not always be possible to reflect electromagnetic signals from the ionosphere between the transmitting and receiving stations. Such is, for example, sometimes the case during nighttime transmissions. A nuclear attack would likely disturb the ionosphere and make it difficult to use. In such circumstances, electromagnetic signals could be reflected between stations from the trails of meteors which strike the earth's atmosphere. Reflecting signals off meteor trails often requires the use of electromagnetic signals within the range of from approximately 30 to 70 MHz.
While operation of an antenna at a single frequency, or over a relatively narrow range of frequencies, may be acceptable for some applications, broadband operation is often desirable. For example, microprocessors are often used in connection with communication systems to select the best frequency and the best available path for transmission at any given moment. However, if the antenna must first be tuned to operate at a different frequency before the transmission can be made, the response of the system is often much too slow to take full advantage of the microprocessor's capabilities. The use of an antenna capable of broadband operation, on the other hand, would allow the communications system to immediately transmit with the optimum frequency and path selected by the microprocessor.
Similarly, in military applications, messages are generally transmitted at several different frequencies, and the signal frequency is often changed in some manner during the transmission. In this way, it becomes much more difficult for unauthorized personnel to intercept the transmitted message, and hostile groups or forces are less likely to be able to jam or distort the transmitted message. In these military applications, therefore, it is critical that the communication system be able to transmit and receive over a broad band of frequencies.
The selection of an appropriate antenna also depends upon whether the communication system must be portable. Portable transmitters and receivers have become indispensable in a wide variety of governmental and commercial applications. For example, portable radio equipment has become an essential tool to police, rescue, and military organizations and to television and radio crews. Also, in military applications, it is highly desirable to be able to move rapidly from one radio transmitting site to another as quickly as possible, and to be able to easily camouflage the antenna. In other applications, it is important to choose an antenna which is less vulnerable to destruction due to shock from a blast, severe weather conditions or other similar circumstances.
Despite the large number of different antenna designs which are currently available, the prior art antenna designs are generally somewhat inflexible. For example, many prior art communication systems employ large antennas which extend high above the earth's surface in order to effectively transmit and/or receive the desired electromagnetic signals. Such antennas are commonly referred to as "aerial" antennas. Typical aerial antennas may, for example, be secured several hundred feet above the earth's surface to the top of a high tower or building; and such antennas are also commonly supported by numerous guy wires which provide the antenna with additional structural stability. It is also quite common to install aerial antennas, together with their supporting towers and guy wires, on the slopes of relatively high mountains. By placing the antennas upon such towers and/or mountains, the range and effectiveness of the antennas can be significantly increased.
Although conventional aerial antennas are generally quite effective and may be constructed so as to operate very efficiently in both transmitting and receiving the desired electromagnetic signals, such antennas are considered "soft" for security purposes. Even though a powerful explosion may be centered some distance away from the above-described aerial antennas, the resulting shockwaves will likely damage or destroy such antennas, thereby rendering the associated communication systems either totally or partially inoperative. Furthermore, aerial antennas which transmit or receive high frequency electromagnetic signals are very susceptible to the adverse effects of the above-mentioned electromagnetic pulse radiation.
Most prior art antenna systems are also inefficient in transmission and reception, except over a relatively narrow band of electromagnetic signal frequencies. Significantly, if such systems are used in connection with military communications which are being transmitted over a number of different frequencies, the antenna system may not be able to efficiently transmit or receive portions of a message. Only those portions of the message which are transmitted at a frequency which is within the narrow operating band of frequencies for which the antenna system was designed can be efficiently transmitted or received. Portions of the message which are transmitted at other frequencies may be either weak or lost entirely.
Some attempts have been made to adapt prior art antenna systems for operation over a broad band of signal frequencies. Such attempts are typically quite cumbersome, however, requiring complex tuning mechanisms or other system adjustments. As a result, few prior art antenna systems have been able to provide the broadband operation characteristics which are needed for many applications.
The prior art antenna systems have also often been very difficult to move from site to site. Frequently, antennas which are reasonably portable are generally less efficient than desirable. Thus, an antenna designer was faced with the choice of designing a very portable antenna or an efficient antenna. Alternatively, in some cases portability could be achieved but the antenna became extremely complex in its construction and deployment, or cumbersome in size and weight. For example, one approach taken in the prior art to provide an efficient portable antenna is to modify the design of a rigid-element antenna intended for use as a permanently installed antenna. This modification generally allowed the antenna to be disassembled for transportation from site to site. Use of rigid elements in a portable antenna also allows the antenna to be of a design similar to a permanently installed fixed-base antenna. Further, the use of a rigid-element antenna generally provides an antenna whose radiation pattern, directivity and standing wave ratio at a particular frequency, is independent of the physical surroundings in which it is operated. There are, however, several problems which accompany the use of rigid-element antennas as portable antennas.
The first of these problems is that the assembly and disassembly of a rigid-element antenna generally takes a significant amount of time and can also be quite complex. The fact that the antenna takes an extended length of time to assemble or deploy reduces its usefulness with a portable transmitter/receiver.
Second, rigid-element antennas, even when disassembled, are often both bulky and heavy, making them difficult to transport. Alternatively, if the weight of the rigid-element antenna is lessened to ease transportation difficulties, the rigid elements of the antenna generally become more fragile requiring greater care in assembly, disassembly, transportation, and use.
Also, a rigid-element antenna generally requires suspension above the ground for proper operation. This is usually done by mounting the antenna on a tall mast. The requirement of a mast further increases the difficulty of transporting and assembling the antenna system, in addition to providing a very conspicuous marking as to the location of the transmitter/receiver. Such conspicuousness can be a great disadvantage in a military operation.
Other types of portable prior art antennas include single-element antennas often consisting of a single vertical element configured in a flexible "whip" manner. A single-element antenna provides some of the required portability in that they are generally easy to transport and assemble. Such antennas are, however, often inefficient transmitters and receivers of electromagnetic signals.
At high and medium frequencies it becomes especially difficult to design an efficient antenna that is still reasonably portable. This can be appreciated by considering, for example, the necessary length of a quarter-wave whip-radiating element that is to be operated at 10 MHz. The shortest length of a whip antenna element which will resonate at a given frequency must be approximately equal to one-quarter the wave length at that frequency. A 10 MHz signal was a wave length of approximately 30 meters. Thus, a quarter-wave antenna must be approximately 7.5 meters in length. It can be appreciated that a rigid antenna element 7.5 meters in length can present considerable difficulties when transported. These problems are compounded when designing a portable antenna for use at frequencies lower than 10 MHz, since the wave length at such frequencies is even longer.